These teachings relate generally to signal amplifiers and, more specifically, relate to variable gain, multi-stage power amplifier bias control circuits and methods, such as those employed in battery powered devices including mobile stations and cellular telephones.
FIG. 1 is a circuit diagram of a dual stage variable gain power amplifier 1 of a type that may be used in a mobile station. The power amplifier 1 is a dual stage device and includes a first stage power amplifier 1A that outputs an amplified signal to a second stage power amplifier 1B. The output of the second stage power amplifier 1B may be connected via suitable coupling circuitry to an antenna (not shown), and is used for transmitting an amplified RF signal over a reverse or return link to a base station or base site for reception and signal processing. The gain of the power amplifier 1 is controlled by the level of a bias control current signal 2 that is input to a current mirror 3 constructed of a first stage current mirror 3A and a second stage current mirror 3B. The current mirror 3 outputs a variable current to a bias current input of each of the power amplifiers 1A and 1B, thereby controlling the gain of each of the power amplifiers in tandem.
A problem that exits in the circuit depicted in FIG. 1 is that the gain collapses at low levels of the bias current (e.g., at a level less than about 40 mA). The gain collapse occurs because of the asymmetry of the power amplifiers 1A and 1B, i.e., the second stage power amplifier 1B is normally sized to be significantly larger than the first stage power amplifier 1A (e.g., a twenty times difference in integrated circuit die area may exist between amplifiers 1A and 1B), and also consumes significantly more current that the first stage power amplifier 1A (e.g., about 80% more current). However, the current mirror 3 is typically symmetrical, and thus provides an equal amount of bias current to each of the amplifiers 1A and 1B. As a result, the current mirror 3 does not correctly proportion the bias current to the first stage power amplifier 1A and to the second stage power amplifier 1B to achieve the lowest possible bias current setting. The end result is that the first power amplifier stage 1A can become starved of bias current before the second power amplifier stage 1B reaches its minimum possible bias current level. Such a collapse in gain of the first stage power amplifier 1A is undesirable, as it can adversely affect the ability of the mobile station to transmit a signal to the base station at low levels of transmitter gain. For example, the collapse in gain of the first stage power amplifier 1A can be manifested as a significant reduction in the input impedance of the amplifier 1A, thus disturbing the circuitry feeding the first stage amplifier.
While it may appear at first glance that one could construct the current mirror 3 in an asymmetrical fashion so that more current was delivered to the first stage power amplifier 1A, in practice this has proved difficult to achieve, and undesirable complexities arise in the design of the integrated circuit that contains the power amplifier 1 and the current mirror 3.
The foregoing and other problems are overcome by methods and apparatus in accordance with embodiments of these teachings.
A method is disclosed for operating a plurality of serially coupled power amplifiers. The method includes controlling the gain of a highest powered power amplifier with a variable bias current Ivariable; and simultaneously controlling the gain of a lower powered power amplifier that feeds the highest powered power amplifier with a bias current having a value equal to Ivariable+Ifixed, where Ifixed less than Ivariable.
A mobile station is constructed to include transmitter circuitry and an antenna for transmitting a signal, where the transmitter circuitry includes a multi-stage power amplifier having a first power amplifier stage with an output coupled to an input of a second power amplifier stage. The output of the second power amplifier stage is coupled to the antenna. The transmitter circuitry further includes a source of variable bias current that is input to the second power amplifier stage for controlling the gain thereof, a source of fixed bias current, and a summing junction for summing the fixed bias current and the variable bias current for input to the first power amplifier stage for controlling the gain thereof. The gain of the first power amplifier stage is controlled such that a minimum desired gain of the second power amplifier stage is achieved without inducing a collapse in the gain of the first power amplifier stage.
The transmitter circuitry may further include a circuit block coupled in series with the source of fixed bias current for selectively turning off the fixed bias current.
The source of variable bias current is coupled to a bias control current signal generated by a data processor that is responsive to required changes in transmitter output power.